使用带有哈希映射的字符串

Question

How would I use a String to specify a specific HashMap .

Eg:

public static ConcurrentHashMap<String, Integer> age = new ConcurrentHashMap<String, Integer>(); 
String s = "age";
s.get("Nick");

Answer 1

If you want to Map a person's name to their age, you need

Map<String,Integer> age = new HashMap<>(); // only use ConcurrentHashMap if you intend
                                           // to use that Map from multiple threads
...
age.put("Nick",42);
...
int nicksAge = age.get("Nick");

Answer 2

public class HashMapTest {
    public static HashMap<String, Integer> ageMap = new HashMap<String, Integer>();

    public static void main(String[] args) {
        ageMap.put("Nick", 12);
        Integer age = ageMap.get("Nick");
        System.out.println(age);

    }

}

Answer 3

You can achieve this using Generics

https://docs.oracle.com/javase/tutorial/java/generics/

Generics allows you to specify a type for a map. So instead of using

public static ConcurrentHashMap age = new ConcurrentHashMap();

You could use

public static Map<String, Short> age = new ConcurrentHashMap<>();

or any other form of map for example

public static Map<String, Integer> ageMap = new HashMap<>();

This simplifies the get and put operations.

Instead of writing a code such as

Object o = ageMap.get("Nick");
if(o instanceof Integer) {
    Integer age = (Integer) o;
}

You could simply write

Integer age = ageMap.get("Nick");